The CDC says 75% of recent measles cases are linked to tight-knit communities, including Russian-speaking groups in Washington.

Measles in Washington — like many recent measles outbreaks in the US — has been spreading in a particular type of community: tight-knit and traditional.

In Washington, the virus has predominantly hit Russian-speaking groups hailing mainly from Ukraine and Russia, according to a source close to the matter. These groups have the lowest rate of vaccination of any population in Washington, the state’s most recent data shows.

And they’re not unique. An ongoing measles outbreak in New York involves mostly people from an undervaccinated Orthodox Jewish community. Since October, 304 people have been sickened by the virus — the highest number in the state in decades.